LaPO4:Ce,Tb based green phosphor material synthesis has been described on several occasions in the prior art. Synthesis methods range from high temperature treatment of individual rare earth salts and oxides with phosphate entities, to the direct co-precipitation of the phosphate at elevated temperatures followed by precipitate firing to form the final material. Regardless of the synthesis method used, it is typically thought by those skilled in the art that the synthesized material must have minimal hard agglomeration, must be close to spherical in shape, have a narrow particle size that is less than 15 microns in its D50 characteristic with high relative brightness in the fresh state and with minimal loss in this brightness when in use. Furthermore, it is typically believed that the CeIII/CeIV and TbIII/TbIV ratios should be maximized in the final form of the LaPO4:Ce,Tb.
To achieve these characteristics, it is generally accepted that co-precipitation is the synthesis method of choice. It is thought that through co-precipitation, the material has better dispersion of the individual rare earths in the host matrix that are claimed to result in higher brightness of the phosphor.
Various methods have been disclosed in the past for forming LaPO4:Ce based phosphors. Co-precipitation of LaPO4:Ce based phosphor was described as early as 1963 by C. W. Struck in U.S. Pat. No. 3,104,226. In this patent, the author describes a method where a mixed rare earth aqueous salt solution that is neutralized to a pH of 5 by a base, such as NH4OH, is prepared. An aqueous (NH4)2HPO4 solution is then added during stirring to form a co-precipitate of lanthanum and cerium phosphates. The resultant precipitate is then filtered, dried and fired at 1,000° C. under a slightly reducing atmosphere to produce the final LaPO4:Ce.
Ropp (U.S. Pat. No. 3,507,804) describes a co-precipitation method used to formulate a rare earth orthophosphate phosphor. A solution of rare earth solubilized in nitric acid is prepared and added over time to an orthphosphoric acid solution. It is disclosed that by keeping the precipitating rare earth slurry concentration at 0.1-6M in rare earth content, acidic, with the rare earth to phosphate molar ratio in the precursor solution between 1:1 and 1:9, and at higher temperatures, one is able to control the phosphor particle size to desired values and to minimize the potential of making large, hard gelatinous agglomerates that result in difficult to mill, inferior phosphors.
Chau et al. (U.S. Pat. No. 5,132,042) describes a non-co-precipitation method of LaPO4:Ce,Tb synthesis that involves the reaction of boron phosphate with La2O3:Ce,Tb precursor. In this patent, it is stated that the boron acts as a slightly reducing agent that aids in the conversion of the CeIV and TbIV to their corresponding 3+ species. This, in turn, is reported to aid in the brightness of the finally prepared phosphor when fired at 1,150° C.-1,300° C.
Collin et al. (U.S. Pat. No. 5,340,556) describes a co-precipitation method of LaPO4:Ce,Tb where the precipitate's brightness is not sensitive to the calcination atmosphere. It is stated that by controlling the precipitation pH at values ≧2 and subsequent maturation treatment in the mother liquor at pH values ≧6, an easily filterable non-gelatinous product is obtained. The control of the pH may be carried out by the addition of a basic compound during the mixing of the solution of the rare earth ions and of the phosphate compound. Thus, if the phosphate is added to the rare earth solution, the basic compound is added simultaneously with the phosphate to control the pH at a value above 2. Similarly, when the solution of rare earth compounds is added to a phosphate solution, the basic compound is added simultaneously to control the pH at a value which is higher than 2 and advantageously constant.
Braconnier (U.S. Pat. Nos. 5,470,503 and 5,746,944) discloses processes wherein a solution of rare earth salts is added to a solution of phosphate ions. In contrast to Collin et al, the patent states that the initial pH of the solution of phosphate ions be below 2 and that the pH of the solution is controlled to values below 2 during the precipitation process by adding a base. An object of the processes is to obtain an improved particle size distribution.
Kimura et al. (U.S. Pat. No. 5,567,403) states that it is a disadvantage to synthesize LaPO4:Ce,Tb with particles that are either ≦1 μm or ≧10 μm. It is claimed that if the particles are too small the phosphor suffers from aging deficiencies and if the particles are too large, they are composed of agglomerates that break creating sub-micron dust that is not advantageous. Furthermore, they state that non-spherical particles present coating problems. To alleviate these potential problems, the inventors disclose a co-precipitation method where the aqueous rare earth salts are added over a limited period of time (3 seconds to 5 minutes) to aqueous phosphoric acid. In doing so, it is stated that control of pH by the addition of base is not necessary. It is also stated that the addition of base to control the precipitation pH as described by other researchers is disadvantageous, in that it has the undesirable effect of forming sub-micron particles.
Collin et al. (U.S. Pat. No. 5,580,490) discloses crystallites of LaPO4:Ce,Tb in the form of “needles”. In this patent, the inventors disclose that when LaCeTb oxalate is mixed with a phosphate anion source, aged in the mother liquor at elevated temperatures and then treated to 600° C. in any atmosphere, hexagonal LaPO4:Ce,Tb with superior brightness is formed. Furthermore, it is stated in the patent that the order of addition of the suspension of rare earth compounds and the phosphate compound is not critical. However, it is preferred to add the phosphate compound to the suspension of rare earth compounds.
All the above synthesis methods for the production of rare earth containing phosphates are described as producing various advantages, such as improved particle size or particle size distribution and brightness that is not affected by calcination temperatures. These methods also are contradictory in that they disclose controlling the pH in different ranges and also that pH should not be controlled. It is apparent that, when considered in totality, the prior art recognizes that adding the rare earth and phosphates according to different regimes can produce rare earth phosphates having differing characteristics.